Plants store energy from sunlight in the form of chemical bonds that compose plants. The energy stored in plant materials can be converted to forms of energy such as heat, electricity and liquid fuels, depending upon the plant material employed and the process applied to extract energy from it. Other processes can produce chemical intermediates from plant biomass that are useful in a variety of industrial processes, for instance lactic acid, succinic acid, etc.
Plant materials have been used for millennia by humans to generate heat by direct combustion in air. For building and process heating purposes, this heat is typically used to generate steam, which is a more transportable heat source used to heat buildings and public areas using heat exchangers of various design. The production of steam may also be used to drive turbines, which transform heat energy into electrical energy. These processes typically involve a simple, direct combustion process of the plant material alone, or a co-firing process with coal or other energy source.
Fuels such as ethanol can be produced from plant materials by a number of different processes. For example, the sucrose in sugarcane can be extracted from the plant material and directly fermented to ethanol using a microorganism, such as brewer's yeast. Brazil has converted a significant portion of its transportation sector over to ethanol derived from sugarcane, proving this can be done on a very large scale over broad geography. As another example, the starch from corn can be processed using α-amylase and glucoamylase to liberate free glucose that is subsequently fermented to ethanol. The US uses a significant portion of its corn crop to produce ethanol from starch. While these advances are significant, the ability to increase the amount of liquid transportation fuel obtained from plant material is limited because only a small fraction of the solar energy captured and transformed into chemical energy in plants is converted into biofuels in these industrial processes.
Plant material can be used for the production of cellulosic biofuels by biochemical processes employing enzymes and/or microorganisms or by thermochemical processes such as Biomass to Liquids (BtL) technology using high temperature and non-enzymatic catalysts. There are also examples of hybrid thermochemical/biochemical processes. Biochemical processes typically employ physical and chemical pretreatments, enzymes, and microorganisms to deconstruct the lignocellulose matrix of biomass in order to liberate the fermentable from cellulose, hemicellulose, and other cell wall carbohydrates, which are subsequently fermented to ethanol by a microorganism. Currently, many different processing methods are being developed for biofuel production that employ different strategies for pretreatment, enzyme cocktails, and microorganisms. Many of these processes are focused on the production of ethanol, but butanol and other useful molecules (e.g., lactic acid, succinic acid, polyalkanoates, etc.) can also be produced in this type of process. The conversion product molecule produced is usually defined by the microorganisms selected for fermentation.
Thermochemical processes employ very high temperatures in a low oxygen (i.e., O2) environment to completely degrade the organic constituents of biomass to syngas, largely composed of molecular hydrogen (H2) and carbon monoxide (CO) gas. These simple molecules are then re-formed into more useful and valuable molecules (fuels or chemical intermediates) utilizing a Fischer-Tropsch process or other methods usually employing a chemical catalyst of some sort. These processes are effective at producing biofuels that are similar to current petrochemical-based hydrocarbon fuels (i.e., gasoline, diesel, jet fuel), although other biofuel molecules can also be produced in these types of processes (i.e., ethanol, butanol, kerosene).
A variant form of thermochemical process uses pyrolysis (i.e., thermal degradation in the complete absence of oxygen) to partially degrade the organic constituents present in plant biomass to a chemically heterogeneous liquid bio-oil. This serves to increase the energy density of the biomass to facilitate transport to centralized processing facilities where the bio-oil is further processed to a desired product slate.
The economic viability of biomass conversion processes is significantly impacted by the composition of the plant material and its conversion efficiency to heat, electricity, biofuels or chemical intermediates under specific processing conditions. For biochemical processes producing biofuels or other chemicals, the recalcitrance of the lignocellulose matrix of the biomass is a major factor in conversion efficiency.